Advanced GIST

INTERVENTION: It is the study of two doses (400mg and 800mg) of STI571 (Imatinib mesylate) to test whether any relevant dose/activity relationship is detectable in patients with unresectable or metastatic gastrointestional stromal tumors (GIST) expressing the kit receptor tyrosine kinase (CD117). Patients will receive study drug until disease progression or unacceptable toxicity. CONDITION: Patients with unresectable or metastatic gastrointestional stromal tumors (GIST) expressing the kit receptor tyrosine kinase (CD117) PRIMARY OUTCOME: To compare the outcome of patients with with unresectable or metastatic gastrointestional stromal tumors (GIST) expressing the kit receptor tyrosine kinase (CD117) treated with low versus high doses of STI571. SECONDARY OUTCOME: To assess response rates of patients with these two doses of STI571. To assess toxicities of patients with these two doses of STI571. INCLUSION CRITERIA: 1. Documented diagnosis of GIST with kit expression. 2. At lease one measureable or evaluable site of disease. 3. Performance status 0,1,2 or 3 (ECOG).